America's bald eagle population has slowly recovered from near devastation over the past 50 years, but now the animals are facing a new threat.
A study published January 13 in the Journal of Wildlife Management finds that lead poisoning from gunshot ammunition is leading to declines in the bald eagle population.
Researchers at Cornell University's Department of Public and Ecosystem Health say poisoning from eating dead carcasses or parts contaminated by lead ammunition has reduced the bald eagle population growth by 4% to 6% annually in the Northeast.
"Hopefully, this report will add information that compels hunters, as conservationists, to think about their ammunition choices," senior author Krysten Schuler told the Cornell Chronicle.
The iconic bald eagles are considered a recovery success in the U.S. after rebounding from near extinction due to widespread use of the insecticide DDT in the twentieth century. Although the population has increased since DDT was banned in 1972, eagles continue to succumb to environmental contaminants, including heavy metals like lead.
Although lead has been banned from things like paint, gasoline and plumbing, it is still widely used in ammunition for hunting. Many hunters "field dress" their kill on the spot, gutting the animal and discarding the organs -- which may contain lead fragments from ammunition -- in the field for scavengers to consume. The issue with that is animals like bald eagles are at risk of being poisoned if they eat the contaminated organs.
Bald eagles aren't the only animals harmed by this practice, the authors point out, as other scavenging species like owls, coyotes and bears can eat the contaminated remains as well.
"We haven't collected data on these other species in the same way that we pay attention to eagles," Schuler told the newspaper. "We're putting eagles out there as a poster species for this issue, but they’re not the only ones being impacted."
To determine the impact lead ammunition has had on the bald eagle population, the research team analyzed population regrowth numbers and necropsy records from 1990 to 2018 from seven northeast U.S. states: Connecticut, Massachusetts, Maine, New Hampshire, New Jersey, New York and Vermont. Researchers then created a computer model to estimate how much the population has declined from deaths linked to lead poisoning.
"We estimated the depression of this growth rate to be 4.2% in female eagles and 6.3% in male eagles," researchers wrote in the study.
Although current levels of lead contamination in the northeast have not caused a region-wide decline of eagles, researchers say the conditions have stressed the resilience of the bald eagle population.
Researchers hope state and federal wildlife managers will use the study results to educate hunters of the consequences of their ammunition choices or to inform policy surrounding the use of lead ammunition.